The present invention relates to a process for producing pitch-based graphite fibers.
Usually, pitch-based graphite fibers are produced by treating pitch fibers in an oxidative gas atmosphere to render them infusible, then heating the infusible fibers up to 800-1,300.degree. C. at a temperature rising rate of 1-30.degree. C. per minute in an inert gas atmosphere, heat-treating the fibers at this temperature for a long time to obtain carbonized fibers substantially free of oxygen, and then heat-treating the carbonized fibers at 2,000.degree.-3,000.degree. C. in an inert gas atmosphere.
The pitch-based graphite fibers thus produced by such conventional process are 40 to 50 TON/mm.sup.2 in terms of elastic modulus, thus exceeding that of polyacrylonitrile-based graphite fibers, while their tensile strengths are in the range of 190 to 220 kg/mm.sup.2 and thus lower than that of polyacrylonitrile-based graphite fibers. In an effort to improve physical properties of such pitch-based graphite fibers, attempts have been made, including improvement of pitch precursor, but they are not considered fully effective. Under the circumstances, it has been desired to establish a method of improving physical properties of pitch-based graphite fibers.
Shortening the graphite fiber producing time is also an important subject from the industrial standpoint. Heretofore, various catalysts and promoters have been studied for shortening the time required for infusibilization treatment, and there have been proposed metal salts, ammonium salts, inorganic acids and halogen. However, in point of the infusibilization promoting effect or physical properties of carbon fibers as the final product, satisfactory results have not been obtained yet. For example, if pitch fibers are subjected to an infusibilization treatment after contact treatment with an inorganic acid such as hydrochloric, sulfuric or nitric acid, physical properties of the final carbon fiber product will be deteriorated.
For increasing the amount of fibers treated, there have been proposed a method (Japanese Patent Publication No. 12740/1976) in which pitch fibers after spinning are deposited on a receiving vessel and then subjected to infusiblizing and calcining treatments, thereafter fibers in form of continuous filaments are drawn out from their deposited state; a method (Japanese Patent Publication No. 37967/1976 and Laid Open No. 90621/1980) in which pitch fibers are deposited on a belt conveyor, then rendered infusible and calcined; a method (Japanese Patent Laid Open No. 6547/1980) in which pitch fibers are suspended from above a bar and rendered infusible; and a method (U.S. Pat. No. 4351,816) in which infusiblized fibers are wound onto a bobbin and carbonized. However, all these methods have merits and demerits. Particularly, because of handling fragile pitch fibers or infusiblized fibers, the fibers are subject to damage, which causes napping or unsatisfactory performance of the fibers after calcination.
According to the prior art, moreover, the carbonization of infusiblized fibers is usually performed by raising the temperature to about 800.degree.-1,300.degree. C. at a rate of 1.degree.-30.degree. C. per minute in an inert gas atmosphere. In this case, it is said that if the temperature rising rate is made higher, it will cause a lowering in strength of the fibers. But, since this conventional carbonizing method requires a high temperature over a long time, it not only causes a lowering of productivity but also it is extremely disadvantageous from the economic point of view.
In the pitch-based graphite fiber manufacturing process, the subject is how to shorten the time required in each of infusibilization, carbonization and graphitization steps, and a calcining step capable of attaining both this shortening of time and improvement of physical properties of product has been considered necessary.